Field-evolved resistance of Rachiplusia nu to Cry1Ac Bt soybean in Argentina

Abstract

Large-scale adoption of genetically engineered soybeans expressing insecticidal proteins from Bacillus thuringiensis (Bt) in South America allow us to understand the ability of pest populations to evolve resistance to Bt crops. Here, we examined the susceptibility of Rachiplusia nu (Guenée) and Chrysodeixis includens (Walker) (Noctuidae: Plusiinae) collected from Cry1Ac Bt and non-Bt soybean fields in Argentina, specifically in the Provinces of Santiago del Estero (Rnu-SE), Córdoba (Rnu-COR), and Tucumán (Cin-TUC), to Cry1Ac toxin. In Cry1Ac-overlay bioassays, the lethal concentration to kill 50% of the larvae (LC50) of Rnu-COR and Cin-TUC populations were 113 and 1,500 ng/cm2, respectively. The lower limit of the LC50 value for Rnu-SE larvae was > 21,679 ng/cm2, at least 14-fold the LC50 value for Cin-TUC larvae and at least 192-fold the LC50 value for Rnu-COR larvae. A similar pattern emerged from the effective Cry1Ac concentrations that inhibited 50% of larval growth. In larval feeding assays using Bt soybean leaves, Rnu-SE insects had higher survival rates than Rnu-COR and Cin-TUC ones. In behavioral assays with Bt and non-Bt soybean plants, the R. nu moths showed a strong oviposition preference for the Cry1Ac Bt over non-Bt soybean plants. These results indicate that a population of R. nu evolved Cry1Ac resistance in Argentina, consistent with the increased number of plusiine loopers causing unexpected damage in Cry1Ac Bt soybean fields. That may correlate with moth oviposition behavior, and further research should guide locally adapted strategies to minimize the resistance drawbacks and maximize the benefits of Bt crops.